1. Field of the Invention
This invention relates to thermal ink jet printing, and more particularly to an improved thermal ink jet printhead.
2. Description of the Prior Art
Generally, a drop-on-demand, ink jet printing system has a printhead that uses thermal energy to produce a vapor bubble in an ink-filled channel in order to expel a droplet. This type of printing is referred to as thermal ink jet printing or bubble ink jet printing and is the subject matter of the present invention. In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359 to Ayata et al, communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The printhead of U.S. Pat. No. 4,463,359 has one or more ink-filled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are shown such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate and thoe having different colored inks for multicolored printing. In one embodiment, a resistor is located in the center of a relatively short channel having nozzles at both ends thereof. Another passageway is connected to the open-ended channel and is perpendicular thereto to form a T-shaped structure. Ink is replenished to the open-ended channel from passageway by capillary action. Thus, when a bubble is formed in the open-ended channel, two different recording mediums may be printed simultaneously.
U.S. Pat. No. 4,275,290 to Cielo et al discloses a thermally activated liquid ink printing head having a plurality of orifices in a horizontal wall of an ink reservoir. In operation, an electric current pulse heats selected resistors that surround each orifice and vaporizes the non-conductive ink. The vapor condenses on a recording medium, such as paper, spaced above and parallel to the reservoir wall, causing a dark or colored spot representative of a picture element or pixel. Alternatively, the ink may be forced above the orifice by partial vaporization of the ink, so that the ink is transported by a pressure force provided by vapor bubbles. Instead of partially or completely vaporizing the ink, it can be caused to flow out of the orifices by reduction of the surface tension of the ink. By heating the ink in the orifices, the surface tension coefficient decreases and the meniscus curvature increases, eventually reaching the paper surface and printing a spot. A vibrator can be mounted in the reservoir to apply a fluctuating pressure to the ink. The current pulse to the resistors are coincident with the maximum pressure produced by the vibration.
U.S. Pat. No. 4,438,191 to Cloutier et al discloses a method of making a monolithic bubble-driven ink jet printhead which eliminates the need for using adhesives to construct multiple part assemblies. The method provides a layered structure which can be manufactured by standard integrated circuit and printed circuit processing techniques. Basically, the substrate with the bubble generating resistors and individually addressing electrodes have the ink chambers and nozzles integrally formed thereon by standard semiconductor processing.
U.S. Ser. No. 719,410, now U.S. Pat. No. 4,601,777, to Hawkins filed Apr. 3, 1985 and assigned to the same assignee as this case, discloses a thermal ink jet printhead and method of fabrication. In this case, a plurality of printheads may be concurrently fabricated by forming a plurality of sets of heating elements with their individual addressing electrodes on one silicon wafer and etching corresponding sets of grooves which may serve as ink channels with a common reservoir in another silicon wafer. The two wafers are aligned and bonded together, so that each channel has a heating element and then the individual printheads are obtained by milling away the unwanted silicon material to expose the addressing electrode terminals and then dicing the wafer into separate printheads.
In all bubble jet or thermal printheads, it is important to be able to keep the ink droplet velocities relatively high and to impart a large momentum to the ejected droplet. This is so, for example, to minimize misdirectionality of the droplet caused by wetting effects at the channel orifices or nozzles and to help overcome first droplet ejection problems in order to assure stable, uniform printing. High droplet velocities and large impulses may be attained by placing the heating element nearer the orifice, so that only a small amount of ink is acted upon the bubble growth and collapse and/or by increasing the heating element current pulse duration to generate more thermal energy, thereby increasing the amount of stored heat in the ink prior to nucleation of the micro-sized vapor bubbles which will lead to a more rapid or explosive bubble growth.
However, in the typical bubble jet printhead discussed above and shown in FIG. 3a, application of one or both of these methods is very limited due to the phenomenon referred to as "blowout." "Blowout" is the mechanism by which a growing bubble within a printhead channel can expand so far as to push out past the channel orifice and release some of the vaporized ink. This occurrence can lead to the ingestion of air into the channel and the possibility of a large trapped air bubble over the heating element surface, as well as a misdirected, weakly propelled droplet. Any trapped air bubble will seriously affect the nucleation process in the ink over the heating elment's surface, as is well known in the art, and cause subsequent misfirings from that channel. This blowout of the growing bubble is due to the lateral spreading of the bubble as it grows. Therefore, placement of the heating element closer to the orifice and/or increasing the heating pulse duration make blowout more likely. Thus, prior art devices accept the lower droplet speeds from less explosive bubble growth to avoid the blowout phenomenon.